CHERNOBYL

Graphite as Moderator

Water is a good moderator but absorbs many neutrons. For the process to work, the nuclear fuel must be enriched before being placed in the reactor (expensive). Graphite absorbs fewer neutrons, so the fuel doesn't need to be as enriched. This is an advantage of the RBMK reactor. In the RBMK reactor, the fuel rods are inserted into a large block of graphite: the moderator. Water flows through channels in the block, carrying away the heat. A disadvantage is the positive void coefficient—more on that later.